DNA Shows It: Birds Are Promiscuous

In one ambitious study, British scientists found that female Blue-Tits prefer having their eggs fertilized by a male other than their social partner.

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Here's the warm and fuzzy part of this column: most birds really do mate for life. But here's the cold side: They mess around.

And here's the switch: Blame the ladies.

Ever since Charles Darwin postulated it would be to a bird's evolutionary advantage to stick with the same mate for its entire life, poets and novelists and even scientists have thought that meant they would remain faithful to the same mate, both sexually and socially.

But that sweet song began to sour a few years ago when scientists, armed with the powerful tools of modern genetics, began capturing birds around the world, and borrowing eggs from active nests, and even following the lives of the hatched chicks to see what was really going on in the avian bedroom.

Females may be socially connected to one male, but they are always on the alert for a better offer, and it frequently comes from the guy next door. In some cases, up to 70 percent of the eggs found in some nests were fertilized by a male other than the primary occupant, protector, and supplier of the nest.

That opens the way for a female that settled for an ordinary chap to enrich the gene pool by inviting a cool dude with obviously very good genes, as shown in his exceptional plumage and long tail, into the bushes with her.

This provides two advantages: greater genetic diversity in her chicks, and thus more resistance to disease, and yet the man of the nest will remain around to help raise the brood, probably unaware that some of the chicks aren't his.

In one ambitious study, British scientists found that female Seychelles warblers prefer having their eggs fertilized by a male other than their social partner. These researchers, from the University of East Anglia, captured more than 97 percent of the warblers on the tiny island of Cousin in the Seychelles. They drew DNA samples from the birds and observed their breeding habits.

Then they monitored the fate of 160 birds that hatched between 1997 and 1999 for 10 years and found that 40 percent of the offspring were fertilized by males other than the female's mate. And most important, these birds had higher genetic diversity of disease detecting genes -- meaning they were more likely to defeat more diseases -- than "if they had been sired by the cuckolded male," the scientists reported in their study, published in the journal Molecular Ecology.

Another study, from the University of Melbourne in Australia and the University of Groningen in the Netherlands, found that "extra pair offspring," as scientists now call birds sired by an outsider, have advantages other than just genetic diversity. These researchers collected 1,732 eggs from 190 blue tit nests to determine their hatching order. All chicks were tested to identify the father.

It turns out that mom gave her illicit chicks a better start in life by hatching the outsider's eggs earlier than her own mate's eggs. That made them stronger than the competition during those crucial early days, and that could give them an advantage for the rest of their lives, the researchers concluded.

"Remarkably, almost 75 percent of the offspring that resulted from these extra pair matings were produced in the first half of the clutch," Michael Magrath of the University of Melbourne said in releasing that study, giving them a 10 hour head start on life.